1. Field of the Invention
The present invention relates to a torque detection assembly for detecting torque without direct contact when external force is applied to a rotating shaft such as a power-steering mechanism of an automobile.
2. Description of the Related Art
In an automotive power-steering mechanism, it is necessary to detect the amount of torque being applied to a steering wheel to determine the amount of power assistance required. Torque detection assemblies for this purpose have been disclosed in Japanese Patent Laid-Open No. 6-174569, for example. The construction of this device will be explained with reference to FIG. 8.
This torque detection assembly includes:
a case 4;
a torsion bar 3 disposed on a central axis of a first shaft 1 attached to a steering wheel (not shown) and a second shaft 2 attached to a pinion gear of a steering mechanism (not shown), the torsion bar 3 being an elastic member connecting the first shaft and the second shaft so as to be elastic in the circumferential direction (the direction of torsion);
a bearing 5 disposed between the case 4 and the first shaft 1, the bearing 5 rotatably supporting the first shaft 1;
a first sleeve 14a composed of a non-magnetic body fastened to the first shaft 1;
a second sleeve 14b composed of a non-magnetic body fastened to the second shaft 2;
a first magnetic element 11 and second magnetic element 12 composed of soft magnetic material fastened to the first sleeve 14a; and
a third magnetic element 13 composed of soft magnetic material fastened to the second sleeve 14b. Teeth 11a are formed in the first magnetic element 11 opposite the second magnetic element 12. Teeth 12a and 13a are formed in the second magnetic element 12 and the third magnetic element 13 opposite each other.
The torque detection assembly also includes:
a first coil 21a disposed around the first magnetic element 11 and the second magnetic element 12;
a first yoke 22a secured to the case 4 so as to surround the outside of the first coil 21a, the first yoke 22a having an internal flange;
a second coil 21b disposed around the second magnetic element 12 and the third magnetic element 13; and
a second yoke 22b secured to the case 4 so as to surround the outside of the second coil 21b, the second yoke 22b having an internal flange.
In order to maintain structural strength, the first to third magnetic elements 11 to 13, the first yoke 22a, and the second yoke 22b are made of a metallic magnetic body having a thickness of 1 to 2 mm, or ferrite having a thickness of 3 to 5 mm. If ferrite, which has low electric conductivity, is used, the magnetic properties are such that highly-sensitive frequency response can be achieved to high frequencies, but because ferrite is extremely brittle and expensive, it is difficult to use in mass-produced goods.
Next, the operation of the above torque detection assembly will be explained. When torque from the steering wheel is applied to the first shaft 1, torsional deformation occurs in the torsion bar 3, and relative angular shear occurs in the circumferential direction between the first shaft 1 and the second shaft 2. Thus, a relative displacement in the circumferential direction occurs between the second magnetic element 12, which is fastened to the first shaft by means of the first sleeve 14a, and the third magnetic element 13, which is fastened by means of the second sleeve 14b, changing the opposing surface area between the teeth 12a of the second magnetic element 12 and the teeth 13a of the third magnetic element 13. Magnetic flux is generated in the second coil 21b by the passage of an alternating drive current, and the magnetic flux passes through a magnetic circuit formed by the second yoke 22b, the second magnetic element 12, and the third magnetic element 13. When the opposing surface area between the teeth 12a and the teeth 13a, which forms a magnetic pathway, is altered, the reluctance of the magnetic circuit changes, changing the inductance of the second coil 21b. The torque is obtained by detecting this change in inductance using a detection circuit (not shown).
Because the second coil 21b allows the generation of eddy currents in the magnetic elements and the yokes, the inductance of the second coil 21b is reduced compared to a hypothetical case in which ideal magnetic elements and yokes which do not generate eddy currents are used. If the magnetic permeability of the magnetic elements and the yoke is constant, then the lower the resistivity, the greater the degree of reduction of the inductance. Because the resistivity of metallic materials is higher at high temperatures, except in special cases, resistivity falls at lower temperatures and the influence of eddy currents increases. Consequently, because the degree of influence of eddy currents depends on temperature, when the assembly is used in environments where temperature fluctuations occur, temperature compensation is required.
Because the first magnetic element 11 and the second magnetic element 12 are both fastened to the first shaft 1 by means of the first sleeve 14a, the relative angular shear between the first magnetic element 11 and the second magnetic element 12 does not change even if torque is applied, and the inductance of the first coil 21a, which is disposed around an intermediate position between the first magnetic element 11 and the second magnetic element 12, does not change. However, because the inductance of the first coil 21a is changed by changes in temperature in the same manner as the second coil 21b, it is possible to obtain an output unaffected by temperature and related only to the torque by detecting the difference in inductance between the first coil 21a and the second coil 21b. 
Now, the voltage generated in the second coil 21b by the passage of the alternating drive current through the second coil 21b is an alternating voltage synchronized with the frequency of the drive current, because output from the torque detection assembly to an ac/dc converter must be in the form of a direct voltage or a direct current proportional to the torque, a low pass filter is required to remove ripples synchronized with the frequency of the alternating drive current and make the output a smooth direct current. Because the drive frequency of the second coil 21b is in the range of a few kHz, the time constant of this low pass filter must be less than a few hundred Hz. Consequently, it is not possible to increase responsiveness to the torque output beyond a few hundred Hz, which is the time constant of the low pass filter.
In recent years, in order to increase responsiveness in automotive power steering mechanisms, a responsiveness of several kHz, an order of magnitude faster than the conventional art, has been sought from torque detection assemblies. In order to meet this demand, it has been necessary to increase the frequency of the drive current by an order of magnitude to between several tens of kHz and 100 kHz.
However, when the frequency of the drive current is increased in a conventional torque detection assembly, the inductance of the second coil 21b and the sensitivity of the inductance to torque is reduced due to the influence of eddy currents, and for that reason, one problem has been that the speed of responsiveness cannot be increased. Because of poor temperature characteristics and variation resulting from eddy currents, another problem has been that it is difficult to achieve complete temperature compensation when operating at low temperatures.
The present invention aims to solve the above problems and an object of the present invention is to provide a torque detection assembly having quick response and superior temperature characteristics.
To this end, according to the present invention, there is provided a torque detection assembly wherein the thickness of a yoke and magnetic elements are not more than twice a skin depth xcex4 calculated by a formula:
xcex4={square root over ( )}(2xcfx81/(2xcfx80.F.xcexcs.xcexc0))
where
xcfx81 is the specific resistance of said magnetic elements,
F is the frequency of the magnetic field,
xcexcs is the specific permeability of said magnetic elements, and
xcexc0 is the permeability of a vacuum.
According to another aspect of the present invention, there is provided a torque detection assembly wherein a yoke is divided into a number of structural elements in a circumferential direction, the structural elements are electrically insulated from each other in portions through which the magnetic flux generated by a coil passes.
According to still another aspect of the present invention, there is provided a torque detection assembly wherein magnetic elements are divided into a number of structural elements in a circumferential direction, the structural elements are electrically insulated from each other in portions through which the magnetic flux generated by a coil passes.